It is known that when aqueous media containing various gaseous molecules of, for example, carbon dioxide and hydrocarbons such as methane, ethane, etc. as dissolved therein are left at a particular temperatures under a particular pressure, they give clathrate hydrates which are crystals where the dissolved gaseous molecules have been trapped in the cages of water molecules. Since such clathrate hydrates are often formed in pipe lines through which oil and natural gas are produced and transported, thereby clogging the pipe lines, they are seriously problematic in safe and continuous operation of plants for oil and natural gas.
Heretofore, in order to inhibit the formation of clathrate hydrates in pipe lines through which oil and natural gas are produced and transported, a large amount of methanol has been added to the pipe lines. However, methanol has extremely high flammability and the harmfulness for human bodies and the environment. In addition, the use of methanol for clathrate hydrate inhibition is also problematic since a part of methanol is lost to the gas phase and at the refinery, it is taken into the propane fraction, and then damages downstream catalysts.
A method of adding large amounts of low-molecular weight substances, such as ethylene glycols, urea and inorganic salts, as clathrate hydrate inhibitors capable of taking the place of methanol to inhibit the precipitation and growth of clathrate hydrates has been proposed. Ethylene glycol or triethylen glycol is sometimes preferred to methanol since it is more easily recovered and does not pollute the gas or condensate (light oil fractions), but they cost more than methanol. Therefore, ethylene glycols can be favorable over methanol only in some cases in which the total production cost is decreased. In addition, such low-molecular weight substances must be added in large amounts, like methanol. Therefore, it was difficult to say that the method is favorable in view of its negative influence on the environment and of the economical aspect. Given the situations, it has been desired to develop other methods of inhibiting the formation of clathrate hydrates for the method of adding methanol and other low-molecular weight substances.
As such other methods of inhibiting the formation of clathrate hydrates for the method of adding methanol and other low-molecular substances, there have recently been proposed various methods of adding water-soluble or water-dispersible macromolecular compounds, such as a method of adding polyethylene glycol or polypropylene glycol (see Japanese Patent Laid-Open No. 6-212178); a method of adding a polyether having cyclic ether structures in its main chain (see EP Laid-Open No. 505000); a method of adding polyvinylpyrrolidone or a copolymer of vinyl pyrrolidinone and .alpha.-olefins (see International Patent Laid-Open No. WO93/25798); a method of adding a copolymer of vinyl pyrrolidinone, vinyl caprolactam and dimethylaminoethyl methacrylate (see International Patent Laid-Open No. WO94/12761), etc.
The most significant difference between the methods of adding such macromolecular compounds and the methods of adding methanol and other low-molecular weight substances lies in the mechanisms inhibiting the formation of clathrate hydrates. Precisely, large amounts of methanol and other low-molecular weight substances are added to systems by which the formation of clathrate hydrates is completely and thermodynamically inhibited or, that is, the formation is completely inhibited due to the freezing-point depression in the systems. Such additives are referred to as thermodynamic inhibitors. As opposed to these, additives of macromolecular compounds are referred to as kinetic inhibitors, which inhibit not completely the formation of clathrate hydrates but retard and inhibit the formation of clathrate hydrates for a certain period of time or control the crystal systems and the sizes of the clathrate hydrates being formed, thereby inhibiting the clogging of pipe lines for a certain period of time. The macromolecular compounds are characterized in that they can exhibit a high inhibiting effect even when added in much smaller amounts than the low-molecular weight substances.
Of such conventional kinetic inhibitors, however, the ability of polyethylene glycol, polypropylene glycol and polyols having cyclic ether structures in its main chain to retard the formation of clathrate hydrates and to inhibit the growth thereof is extremely poor. The ability of polyvinyl pyrrolidinone and copolymers of vinyl pyrrolidinone and a -olefins to retard the formation of clathrate hydrates and to inhibit the growth thereof is higher than that of polyethylene glycol, polypropylene glycol and polyethers having cyclic ether structures in the main chain but is not satisfactory. The ability of the copolymer of vinyl pyrrolidinone, caprolactam, and dimethylaminoethyl methacrylate, on the other hand, to retard the formation of clathrate hydrates and to inhibit the growth thereof is higher than the other, above mentioned macromolecular compounds, however, its performance is not yet good enough and the conditions under which the polymer is applicable are limited.